It is known that materials such as MnBi, MnAlGe, GdFe, TbFe and GdCo may be used in a recording medium, for example a magnetooptical disk, for use in an optical recording system. These materials are usually deposited or sputtered in a vacuum on a glass substrate, a silicon plate or the like in order to provide a thin film for magnetooptical recording medium. Magnetooptical recording media having the following properties in common:
1. the easy axis of magnetization is perpendicular to the surface of the film, and
2. their magnetic transition temperature (Curie temperature and compensation temperature, respectively) are comparatively low.
Since the easy axis of magnetization is in a direction perpendicular to the surface of the film, binary information can be recorded in the form of "1"s and "0"s represented by locations in the film magnetized upwardly or downwardly. Hence the device may be used as a digital memory.
A method of thermomagnetically recording information, known as Curie point writing, may be carried out as follows. First, the whole film is magnetized in the downward direction or is set at "0". When the information "1" is to be recorded, a laser beam is directed onto the location at which the information is to be stored while simultaneously applying an upwardly directed external magnetic field. After the irradiated location is heated to a temperature near the Curie temperature by the laser energy, the magnetization of the external field is "trapped" upon cooling. the magnetization of the irradiated location remains upwardly directed so that the information "1" is recorded.
When the information "0" is to be recorded, the laser beam is not used because all locations on the film are initially magnetized "0". When a location which is not irradiated by the laser beam is kept at a temperature sufficiently below the Curie temperature or has a sufficiently large coercive force H.sub.c, its direction of magnetization will not be reversed by applying an external magnetic field, so that the downward magnetization (information "0") is maintained.
When the recorded information is read, a plane polarized light beam is used to scan the recording medium. Depending on whether the magnetization in a scanned location (bit) is directed "up" or "down", the plane of polarization of the reflected beam is rotated in one or in the other direction (so-called magnetooptical reading).
In order to make use of the present thermomagnetic recording systems with magnetooptical reading attractive, certain conditions should be satisfied both as regards writing and as regards reading.
In order to obtain a sufficient signal-to-noise ratio, the magnetooptical figure of merit, which depends on the magnetooptical angle of rotation and the absorption coefficient, must be as large as possible. When the above-mentioned metallic magnetooptical materials are used, the magnetooptical figure of merit in reflection is comparatively low, however, because the magnetooptical rotation in reflection (the Kerr rotation) is comparatively small. The magnetooptical figure of merit in transmission is also small because the absorption is high.
A higher magnetooptical figure of merit can be obtained by using magnetooptical material based on garnet, which in transmission in a given wavelength range exhibit very high Faraday rotations in combination with low absorptions. However, as a result of such low absorption, this kind of material has difficulty in the writing process. The low optical absorption results in a poor coupling of light in the film, i.e. the energy absorption is small. As a result, in order to heat the layer to the Curie temperature, much more laser power is necessary than is the case with metallic layers of comparable thicknesses. Although the coupling can be increased by making the layer thicker, very small bits cannot be written in thick layers. In that case the recording density is reduced.